V5856 Sagittarii/2016: Broad Multiepoch Spectral Coverage of a Sustained High-luminosity Nova

Nova V5856 Sagittarii is unique for having remained more than nine magnitudes above its pre-outburst brightness for more than 6 yr. Extensive visible and infrared (IR) spectra from the time of outburst to the present epoch reveal separate emitting regions with distinct spectral characteristics. Permitted emission lines have both broad and narrow components, whereas the forbidden line profiles are almost entirely broad. The permitted line components frequently display P Cygni profiles indicating high optical depth, whereas the broad components do not show detectable absorption. The densities and velocities deduced from the spectra, including differences in the O i λ7773 and λ8446 lines, are not consistent with an ongoing wind. Instead, the prolonged high luminosity and spectral characteristics are indicative of a post-outburst common envelope that enshrouds the binary, and is likely the primary source of the visible and IR emission.